Distributed processing systems such as large digital telecommunication switching systems may have a number of independently operating microprocessors of different types to perform various functions of the system. Each of the microprocessors has to be programmed and tested. Some testing may be done by emulation on a general purpose computer, but ultimately, a new program has to be integrated with the hardware and software structure of the microprocessor and tested as part of the integrated system. To perform integration testing of the new program of a microprocessor, a test access is required.
One prior art approach is to use in-circuit emulation whereby a special processor adapted for testing is inserted in the system in the place of the processor of the system. This has obvious drawbacks in that the program is not tested on the processor on which it will ultimately be run. Another prior art approach is to provide an interface circuit to the microprocessor on which the program will be run in order to provide stimuli to the microprocessor and to observe its reaction. Such an interface circuit is connected to the peripheral bus of the microprocessor to be tested and is provided with circuitry which assures compatibility between the test apparatus and the microprocessor to be tested. However, there are a number of different types of microprocessors made by a variety of manufacturers which have different characteristics and under the known prior art schemes, a new interface must be designed for each microprocessor to be tested in order to assure compatibility between the diagnostic apparatus and the unit to be tested.